1. Field of the Invention
The present invention relates to a disc recording medium having a function of error-correcting data to be recorded on the medium and an apparatus for reproducing the data from the disc recording medium.
2. Description of the Related Art
In a case that a recording medium such as an optical disc contains defects or dust adhering to a recording plane of the disc, an error takes place on the data to be reproduced from the recording medium. This error makes the servo operation unstable and thereby brings about a further data error. In general, the data recorded on the optical disc is recorded together with parity bits used for error correction.
For example, consider an optical disc such as a digital video disc whose recording capacity is on the several giga bytes level. This kind of optical disc is arranged to perform error correction about each unit data of 32 KB. This error correction unit is referred to as an ECC block.
FIG. 1 is a model view showing a format of an ECC block of the optical disc. This ECC block is composed of a two-dimensional array consisting of 172 words.times.192 rows and contains parity bits added in two error-correcting systems. The format of this ECC block is arranged to add parity bits for ten words of 172 words in the direction of the bit stream of the data, that is, the direction of C1 shown in FIG. 1. (These parity bits in the C1 direction are referred to as a PI parity.) The format of this ECC block is arranged to range 192 rows each of which is composed of 172 words and the PI parities corresponding to ten words and add parity bits for 16 words in the direction perpendicular to the bit stream, that is, in the C2 direction shown in FIG. 1. (These parity bits in the C2 direction are referred to as a PO parity.) These PI and PO parities are used when the data is error-corrected by a RSPC (Reed Solomon Product Code).
On the other hand, in recent days, an optical disc is now being required for enhancing its density, for the purpose of recording a larger capacity of data than the digital video disc. In order to enhance the density of the optical disc, it is necessary to keep a spot size of a laser beam to be projected onto the recording medium smaller than the conventional size.
The spot size of the laser beam has a radius R expressed as shown in the following expression (1). EQU R=0.32.lambda./NA (1)
where NA denotes a numeric aperture and .lambda. denotes a wavelength of the laser beam.
As represented in this expression (1), for reducing the radius R of the spot size of the laser beam in size, it is necessary to shorten the wavelength .lambda. of the laser beam or enlarge the numeric aperture NA of an objective lens.
When the numeric aperture NA of the objective lens is made larger, however, the aberration is made so large that the signal cannot be recorded or reproduced from the recording medium. As one of the solving methods, it is known that the disc substrate is made so thin that the aberration becomes lower.
If the thickness of the disc substrate is made thinner, as mentioned above, the aberration is lower. However, small dust left on the surface of the disc substrate brings about data error though such small dust conventionally has no influence on the recording and reproduction of the optical disc.
FIG. 2 is a graph showing an error propagation of an error against a diameter of dust left on the surface of the disc substrate as the thickness of disc substrate is changing. In this graph, an axis of abscissa denotes a diameter of dust left on the surface of the disc substrate, while an axis of ordinates denotes a propagating distance of error, in which the unit is a micrometer order. The error propagating length is calculated on the assumption that an error takes place when an amplitude of a RF signal output from a photo detector to which the reflected laser beam is applied is 55% or less. It is grasped from the graph shown in FIG. 2 that no error takes place if a dust diameter is about 300 micrometer or less when the thickness of the disc substrate is 1.2 mm. This thickness corresponds to the thickness of the so-called compact disc. Further, it is also grasped from the graph of FIG. 2 that no error takes place if the dust diameter is about 150 micrometer or less when the thickness of the disc substrate is 0.6 mm. This thickness corresponds to the thickness of the so-called digital video disc.
Further, when the thickness of the disc substrate is 0.3 mm, the error propagating length is 200 micrometer when the dust diameter is about 100 micrometer. When the thickness of the disc substrate is 0.15 mm, the error propagating length is 60 micrometer when the dust diameter is about 20 micrometer. Further, when the thickness of the disc substrate is 0.02 mm, the error propagating length is several tens micrometer when the dust diameter is about several micrometers.
As mentioned above, this graph has indicated that even dust of such a diameter as giving no influence when the disc substrate has a conventional thickness such as 1.2 mm or 0.6 mm is likely to have influence on the data recording or reproduction. Hence, even small dust left on the surface of the disc substrate brings about a data error though it does not conventionally have any influence on the data recording and reproduction.